In certain technical applications, for example but not exclusively printers, heaters or lamps, there is a need for AC-AC converters which are able to supply output voltages of variable amplitudes which otherwise follow the sinusoidal waveform of the utility mains, especially in terms of phase and frequency.
There are techniques such as phase angle control or integral cycle control which are typically used in alternating current networks to provide regulated output voltage.
The disadvantage with the phase angle control method is that the utility mains current contains harmonics which are not present in the utility mains voltage. Other loads, which are connected to the utility mains, suffer from these negative effects of the harmonics generated. In order to filter these harmonics, one needs to use very large filter circuit components making the overall solutions bulky and causing more electric losses. The European Standards EN61000-3-2 and EN61000-3-4 regulate the necessary limitation of mains harmonics.
The disadvantage with the integral cycle control is that it utilizes full-wave or half-wave packet control and loads the utility mains network periodically. This periodic loading of the utility mains network creates fluctuations in the utility mains voltage producing the so-called flicker effect. The European Standard EN61000-3-3 regulates the necessary limitation of mains flicker.
Typical loads which can cause problems in terms of harmonics and flicker to power networks are heating elements formed by ohmic resistors or inductive loads as present in motors or filter circuits. Such loads can also be a printer power supplies which may be not fulfilling the safety standards and so cannot be directly connected to mains and therefore require an isolated power source. Also, such printer loads are typically fast changing and need a regulation that should ideally be able to provide the steady output voltage even during the printing of pages.
Especially for such printers, there is a need to produce an output voltage, which does not create harmonics and flicker, thereby reducing the filter size and the losses. Also what is needed here is to produce an isolated output voltage which is regulated, synchronized with the utility mains voltage and has the same frequency as the utility mains voltage.